Ultrasonic transducer probe

ABSTRACT

An acoustic generator, comprising: a source of electro-magnetic radiation; a waveguide coupled to said source; and at least one absorbing region defined in said waveguide, said region being selectively absorbing for portions of said radiation meeting at least one certain criterion and having significantly different absorbing characteristics for radiation not meeting said criterion, both of said radiation portions being suitable for conveyance through said waveguide, wherein said absorbing region converts said radiation into an ultrasonic acoustic field. Optionally, said region comprises a volumetric absorber. Alternatively or additionally, said region comprises a plurality of regions.

FIELD OF THE INVENTION

The present invention relates to the field of probes includingultrasonic transducers that are powered and/or controlled usingnon-electrical transmission methods.

BACKGROUND

Small cross-section catheters having ultrasound capability at oradjacent to their tips are known in the art. However, transmission ofelectrical power and/or signals through such thin catheters challengesthe design and constrains the ability to reduce the cross-section of thedevices. Consequently, several suggestions to transmit power to (andreceive signals from) the tip of the catheter using optical waves andconvert the optical waves into ultrasonic waves using a suitabletransducer, are recorded in the art.

The phenomenon of conversion of electro-magnetic radiation to ultrasoundis well established. Of the different conversion modes ofelectro-magnetic radiation to ultrasound conversion in thethermo-elastic regime is of primary, but not solitary, interest in thisdescription. In the thermo-elastic regime, a portion of theelectro-magnetic radiation absorbed in a target material heats up aregion within the target material. Provided the rate of heat depositionis larger than the rate of its dissipation away from the radiatedregion, the region experiences an increase in its temperature. Theresulting thermal stress generates an acoustic disturbance propagatingaway from the heated region. The rate of heat deposition, as determinedfrom the temporal and spatial parameters of the irradiation wavefront,the rate of dissipation of the heat away from the heated region, and thespatial distribution of temperature in the heated region and thephysical properties of the target material determine the characteristicsof the resulting acoustic signal.

U.S. Pat. No. 5,944,687, the disclosure of which is incorporated hereinby reference, uses a transducer comprising a fluid reservoir at the tipof the catheter. The fluid is heated by a pulse of laser lighttransmitted through the catheter. When the heated fluid expands itcauses a cap (or bellows) on the fluid reservoir to move. Theillumination is transient, and after the light is interrupted, the fluidcontracts and the cap retracts.

U.S. Pat. No. 6,022,309, the disclosure of which is incorporated hereinby reference, describes a different implementation, in which workingfluid is conveyed to outside the catheter. Once outside, the fluid isirradiated with pulsed laser light and converts the laser light intoultrasound radiation. Therefore, the ultrasound radiation is generatedoutside the confines of the catheter.

U.S. Pat. No. 5,254,112, the disclosure of which is incorporated hereinby reference, describes a catheter in which pulsed laser light hits atarget that allegedly generates ultrasound radiation in a directionperpendicular to the target's surface, counter-incident to the lightenergy. The targets described are metallic. This catheter can allegedlyalso transmit a high power laser, that is reflected to propagate in thesame general direction as the ultrasound radiation, to optically ablateplaque in the vicinity of the catheter. The patent claims that thedirection of the acoustic radiation is at a near-right-angle, slightlyproximal, to the axis of the catheter. How this happens is not, however,described by the instant applicant. This patent also describes detectionof acoustic radiation at the probe by detecting its interaction with anoptical signal (e.g., using a laser beam) that is also introduced to theprobe tip. A single fiber may run along the catheter and be used,apparently selectively, for conveying ultrasound generating laser lightand for detecting acoustic radiation, by using a selectively reflectingsurface that passes ultrasound generating radiation and reflectsultrasound detecting radiation. Acoustic interaction between ambientultrasound waves and sensing light is with a transparent interposingmedium between the fiber and the reflector. This patent apparently doesnot suggest using a same fiber simultaneously for more than onefunction.

This patent uses a multi-fiber catheter, with each fiber being used toselect one angular segment and transmit light and/or ultrasonic energyin a direction generally perpendicular to the catheter axis. Also, acentral guidewire is used to guide the catheter. Thus, this designnecessarily requires a significantly larger diameter than a catheterutilizing a single fiber.

In addition, the power of the ultrasound generated by this patent isapparently constrained by several fundamental loss processes: (a) mostof the powering laser light is apparently lost by reflection from themetallic target, some into surrounding tissue (with an added potentialhealth hazard), and (b) most of the resulting ultrasound is apparentlydissipated within the construction of the catheter. The later effectreduces the effectiveness of the system both in the introduction ofuncontrolled ultrasonic signals that introduce large backgroundinterference that severely compromises the performance of the device aswell as in a significant reduction in the available power. In addition,unwanted power is apparently also absorbed by the surrounding tissue.

SUMMARY OF THE INVENTION

An aspect of some embodiments of the invention relates to a method ofgenerating ultrasonic radiation from electromagnetic radiation. In anexemplary embodiment of the invention, a waveguide for theelectromagnetic radiation includes one or more absorbing regions thatselectively absorb a portion of the radiation, said selection optionallyeffected by discrimination on the basis of wavelength and/orpolarization. A pulse (or train of pulses) of radiation is transmittedtowards the absorption region and causes the absorbing regions to expandabruptly, generating ultrasonic radiation. In an exemplary embodiment ofthe invention, the waveguide is an optical fiber and the absorbingregions are defined in or on the core of the fiber. Alternatively, theabsorbing regions are segments that are added to the fiber. Optionally,the waveguide is terminated by an absorbing region. An absorbing areamay be thin or a boundary layer, for example, a thin layer of metal orother material, especially a dichroic material or a wavelength selectivereflective element such as a grid.

In an exemplary embodiment of the invention, a guidewire for medicalapplications comprises a single wave-guide, such as an optical fiber,with a wavelength-selective absorbing region at its end. When laserlight of that wavelength is pulsed through the fiber, the absorbingregion generates acoustic radiation. Optionally generation is bythermo-elastic generation, in which thermal stresses are introduced as aresult of the absorbed light. Optionally, light of a second wavelengthis transmitted substantially unhampered through the fiber, for example,to exit past the absorbing region. Alternatively or additionally, areflector is provided at the end of the fiber, to reflect the light ofthe second wavelength back, with the phase, frequency, polarizationand/or amplitude of the light being affected by an optical-acousticinteraction at or near the reflector. Optionally, such interactions areused for detecting an acoustic field. Alternatively or additionally, areflector is provided at the end of the fiber, to reflect the light ofthe absorbing wavelength back, so as to even out the temperaturedistribution due to the absorption in the absorbing region.

In an exemplary embodiment of the invention, the absorbing region isdense enough to absorb all the intensity of the incident radiation, suchthat no portion of the absorbed wavelength is transmitted past theabsorbing region. Alternatively, a portion of the energy at the absorbedwavelength is transmitted through the region, while another portion isabsorbed. Alternatively, multiple absorbing regions are provided for asame wavelength, with each region absorbing some light and transmittingsome light. The absorbency of the regions may be designed to provide auniform (or shaped) thermal distribution so as to generate a specificform of ultrasonic field.

In an exemplary embodiment of the invention, different absorbing regionsare provided for different wavelengths. Optionally, one terminatingregion is provided to absorb all relevant wavelengths. Optionally, thereis a spatial overlap between absorbing regions for differentfrequencies, for example a 0.1 mm region that absorbs a first wavelengthincludes a 0.05 mm sub-region that absorbs a second wavelength inaddition to the first wavelength. Such overlap potentially increases thedesign flexibility in controlling the acoustic transmission envelope,direction and/or frequency.

In an exemplary embodiment of the invention, the selectivity of theabsorbing area is relative to the wavelengths that the waveguide caneffectively transmit. For example, the total wavelength range of thewaveguide may be divided into sub-ranges, each being selectivelyabsorbed by a certain material. For example, two, three, four or moredifferent ranges may be provided. Alternatively or additionally, theselectivity is relative to the separation possible with the laser sourceused, for example, a tunable laser or a multiple laser source, e.g.,with wavelength divisions of 100 GHz or less.

In an exemplary embodiment of the invention, the waveguide is used toguide the radiating energy to ensure that most or all of the energypasses through the (one or more) absorbing region. Thus, beam expansionand diffraction problems can be avoided.

An aspect of some embodiments of the invention relates to the generationof ultrasound by the absorption of electromagnetic radiation by anabsorbing solid volume. Optionally, the absorbing solid is lightlyabsorbing such that the absorption is gradual along the direction ofpropagating of the radiation, rather than the energy being absorbed on asurface or boundary layer of the volume. Optionally, the absorbingvolume is inserted into the body and used for treatment and/or imaging.Optionally, the volume is selectively absorbing of wavelength,polarization and/or does not block the entire cross-section of a lightguide used to provide the light.

In an exemplary embodiment of the invention, a reflector is provideddistal of an absorbing region, to reflect radiation that is not absorbedby the region on the forward pass, back into the same region for furtherabsorption. Optionally, the radiation is made to reverberate severaltimes through the absorbing region. This can be accomplished, forexample, by two reflectors, positioned on either side of the absorbingregion. Alternatively a polarization-based two-pass reflecting systemcan be implemented by providing a polarization changing element at thedistal reflector and/or at the entrance to an absorbing area (orintegrated into the absorbing area), so that the radiation inside theabsorber has a polarization that is reflected by a polarizationdependent reflector provided at the entrance to the absorbing volume.Such a polarization dependent reflector may also be provided at the exitfrom the absorbing volume. Optionally, the reflector(s) and/or thenumber, size and/or density of the absorbing volume(s) are selected tocontrol the uniformity of the waves generated by one or more regions. Aparticular region may include absorber density variations along itslength and/or cross-section, alternatively or additionally to changes inwavelength-dependent behavior.

In an exemplary embodiment of the invention, multiple absorption regionsare placed along the wave-guide. The type, dimensions and relativepositions of these regions may be used to determine the characteristicsof the generated ultrasound. Suitable arrangements can optionallydetermine the directionality, spectral contents, waveform, and theintensity of the ultrasonic radiation. A potential benefit of multipleor extended regions is better heat dissipation, possibly allowing higherultrasonic peak-power to be effectively used.

In an exemplary embodiment of the invention, a plurality of absorbingregions act in concert to provide a desired energy field distributionand/or wave propagation direction. For example, the distance between twoabsorbing regions may be related to a desired acoustic wavelength to begenerated. The absorbing regions that act in concert may be absorbing asame wavelength of radiation or different wavelengths. Alternatively oradditionally, the number, spacing and/or length of the regions may beused to select the wavelength spectrum generated in one or moredirections. Alternatively or additionally, the regions in a same ordifferent fiber may be used to steer the ultrasonic waves, for example,using phase differences between the regions.

In an exemplary embodiment of the invention, a plurality of absorbingregions are used to generate a strong acoustic wave while maintaining alow average acoustic radiation power, which radiation power is desirablybelow a break-down point of the absorbing target. The plurality ofabsorbing regions allows the target to accumulate a larger overallacoustic power while maintaining the peak power level at each regionbelow a specified threshold.

In an exemplary embodiment of the invention, the ultrasound is generatedwithout any free-space propagation of light, with light going directlyfrom a wave-guide to an absorbing volume. Alternatively, spaces aredefined in the waveguide, for example if the waveguide is hollow or byproviding air (or vacuum or other fluids or gasses) spaces, such asexpansion spaces, adjacent the target.

An aspect of some embodiments of the invention relates to control ofultrasound properties by spatial and density design of absorbingvolumes. In an exemplary embodiment of the invention, the controlincludes one or more of uniformity, frequency, number of cycles,directivity and waveform. In an exemplary embodiment of the invention,the control is achieved by providing multiple and suitably spacedabsorbing volumes, possibly with different volumes being addressableusing different wavelengths, polarizations and/or via different fibers.Alternatively or additionally, the volumes have controlled densities,which may be matched, for example, to the expected relative intensity ofa electromagnetic wave at the volume. It should be noted that thiscontrol contrasts with that suggested in the art for fluid basedsystems, in which the absorption depth is fixed and a single volume isused. While the use of solids is desirable in many embodiments of theinvention, other material phases, such as gas or liquid may be used. Inthe example of absorption outside of a catheter, the density ofabsorbing material may be controlled in order to achieve a desiredradiation volume.

An aspect of some embodiments of the invention relates to providingmultiple absorbing regions in a waveguide, for generation of ultrasoundfrom each of the regions.

An aspect of some embodiments of the invention relates to providingmultiple electro-magnetic radiation waves in a wave-guide, such that aplurality of functions are provided. The multiple waves may havedifferent polarization and/or wavelengths. In an exemplary embodiment ofthe invention, one of the waves is used for the generation of ultrasoundand another wave is used for detection of ultrasound or treatment basedon the radiation. Such treatment may be, for example, treatment usingthe radiation, treatment using heat or treatment using high poweredultrasound generated from the radiation. In an exemplary embodiment ofthe invention, ultrasound radiation is generated from theelectromagnetic wave during forward traveling of the electro-magneticwave.

An aspect of some embodiments of the invention relates to anacousto-optical medical probe that provides forward directed ultrasonicradiation and forward directed light radiation. Optionally, forwardlooking ultrasonic detection is provided as well. Alternatively oradditionally, side-looking ultrasound radiation, side-looking lightradiation, and/or side-looking ultrasonic detection may be provided.Alternatively, ultrasound detection and/or generation may be by anexternal probe. In an exemplary embodiment of the invention, theacoustic radiation and light radiation are provided using a same opticalfiber.

An aspect of some embodiments of the invention relates to steering anultrasound beam using a plurality of acousto-optical sources. In anexemplary embodiment of the invention, the sources are provided indifferent fibers or in different (possibly partially overlapping) partsof a cross-section or a length of a same fiber. In an exemplaryembodiment of the invention, the relative phase in the different partsis controlled by providing suitable radiation to the sources. Thedirection and/or angle of view of the beam is set using phase and/orintensity differences between the different sources. Optionally, thephase differences are controllable by modifying the timing and/or otherproperties of the source radiation.

There is thus provided in accordance with an exemplary embodiment of theinvention, an acoustic generator, comprising:

-   -   a source of electro-magnetic radiation;    -   a waveguide coupled to said source; and    -   at least one absorbing region defined in said waveguide, said        region being selectively absorbing for portions of said        radiation meeting at least one certain criterion and having        significantly different absorbing characteristics for radiation        not meeting said criterion, both of said radiation portions        being suitable for conveyance through said waveguide,    -   wherein said absorbing region converts said radiation into an        ultrasonic acoustic field. Optionally, said criterion comprises        wavelength such that said absorbing region is wavelength        selective. Alternatively or additionally, said criterion        comprises polarization such that said absorbing region is        polarization selective. Alternatively or additionally, said        generator is adapted to be inserted into a body. Alternatively        or additionally, said waveguide comprises an optical fiber.        Optionally, said fiber includes a non-acoustic optical fiber        sensor. Alternatively, said absorbing region comprises a segment        that is added to said fiber. Alternatively, said absorbing        region comprises a doping of a core or damage to the core of        said fiber.

In an exemplary embodiment of the invention, said absorbing region isoptically controllable to change at least one of said criterion and itsabsorption. Alternatively or additionally, said source comprises a lasersource. Alternatively or additionally, said source comprises a couplerfor a laser source. Alternatively or additionally, said source comprisesa spectral filter.

In an exemplary embodiment of the invention, said at least one absorbingregion comprises at least two absorbing regions. Alternatively oradditionally, said at least one absorbing region comprises at leastthree absorbing regions. Alternatively or additionally, said at leastone absorbing region comprises at least four absorbing regions.

In an exemplary embodiment of the invention, said at least two regionshave same absorbing characteristics. Alternatively or additionally, saidat least two regions have different absorbing characteristics.Alternatively or additionally, said at least two regions have at leastone different absorption selectivity criterion. Alternatively oradditionally, said at least two regions have same selectivity.Alternatively or additionally, the absorption properties of said atleast two regions are adjusted so as to achieve a desired effect on saidultrasonic waves. Alternatively or additionally, said at least tworegions are spaced apart to achieve a desired effect on said ultrasonicwaves. Optionally, said effect is selection of a wavelength spectrum.Alternatively or additionally, said effect is a selection of a spatialfield distribution. Alternatively, said effect is a selection of anacoustic envelope shape.

In an exemplary embodiment of the invention, said absorbing region is avolume absorber that absorbs said radiation along its length in adirection of propagation of said radiation. Optionally, said absorbingregion has axially uniform absorption characteristics, along the axis ofsaid waveguide. Alternatively, said absorbing region has axiallynon-uniform absorption characteristics, along the axis of saidwaveguide. Alternatively, said absorbing region has stepped absorptioncharacteristics, along the axis of said waveguide.

In an exemplary embodiment of the invention, said absorbing region is asolid absorber. Alternatively, said absorbing region is a fluidabsorber.

In an exemplary embodiment of the invention, said waveguide comprises anacousto-optical modulator portion that modulates light waves responsiveto an acoustic field. Optionally, the generator comprises an opticaldetector coupled to said waveguide which generates a signal responsiveto said acoustic field. Optionally, said optical detector detectsradiation that passes through said absorbing region unabsorbed.Alternatively or additionally, the generator comprises a signalprocessor that reconstructs an image from said signal. Optionally, saidimage is a one dimensional image. Alternatively, said image is a twodimensional image.

In an exemplary embodiment of the invention, the generator comprises asignal processor operative to reconstruct a tissue characterization fromsaid signal. Alternatively or additionally, the generator comprises asignal processor operative to reconstruct a distance from said signal.

In an exemplary embodiment of the invention, said source provides a highpower laser beam that passes through said absorbing region substantiallyunabsorbed.

In an exemplary embodiment of the invention, said selectivity providesselectivity of at least two different criteria of wavelengths that canpass through said waveguide.

In an exemplary embodiment of the invention, said selectivity providesselectivity of at least three different criteria of wavelengths that canpass through said waveguide.

In an exemplary embodiment of the invention, said generator comprises aplurality of waveguides arranged in a phased-array and a controller thatcontrols said source to activate said array as a phased-array.

In an exemplary embodiment of the invention, said ultrasonic wave isoperative to be steered in space by said generator without moving theabsorbing region.

In an exemplary embodiment of the invention, said generator comprisesonly a single waveguide.

In an exemplary embodiment of the invention, said generator comprises anultrasonic absorber, which spatially shapes said ultrasonic waves.

In an exemplary embodiment of the invention, said generator comprises acontroller operative to control said source. Optionally, said controllersynchronizes an operation of said generator with a separate treatmentdevice. Alternatively or additionally, said controller synchronizes anoperation of said generator with a separate imaging device.Alternatively or additionally, said controller reads out optical signalsreceived via said waveguide.

There is also provided in accordance with an exemplary embodiment of theinvention an acoustic generator, comprising:

-   -   a source of electro-magnetic radiation;    -   a waveguide coupled to said source; and    -   at least one volumetric absorbing region defined in said        waveguide, which absorbs radiation along its length in a        direction of propagation of said radiation,    -   wherein said absorbing region converts said radiation into an        ultrasonic acoustic field. Optionally, said absorber is        uniformly absorbing along its length. Alternatively, said        absorber is non-uniformly absorbing along its length.        Optionally, said non-uniformity is designed to achieve a certain        absorption profile. Optionally, said absorption profile is        designed to achieve a substantially uniform energy deposition        along said absorber.

In an exemplary embodiment of the invention, said non-uniformity isstepped, defining a plurality of contiguous uniform sub-regions withdifferent absorbing characteristics.

Optionally, said non-uniformity is stepped, defining a plurality ofnon-contiguous uniform sub-regions with different absorbingcharacteristics.

In an exemplary embodiment of the invention, said generator comprises areflector for reflecting at least a portion of the light that passesonce through said absorber, to pass at least a second time through saidabsorber. Optionally, said generator comprises a second reflector forreflecting at least a portion of the light that passes twice throughsaid absorber, to pass at least a third time through said absorber.Alternatively, said second reflector is polarization discriminating andsaid generator comprises a polarization rotator.

In an exemplary embodiment of the invention, half a thickness of saidabsorption area absorbs less than 80% of light absorbed by saidabsorbing area.

In an exemplary embodiment of the invention, said absorbing region has anon-uniform cross-section.

In an exemplary embodiment of the invention, said absorbing region doesnot fill a cross-section of said waveguide.

In an exemplary embodiment of the invention, said waveguide guidessubstantially all radiation provided in waveguide to said absorbingregion. Optionally, said guidance comprises guiding said radiation tohave a substantially uniform cross-section along said absorbing region.

In an exemplary embodiment of the invention, said absorbing regionselectively absorbs only some of said radiation.

In an exemplary embodiment of the invention, said generator comprises aplurality of absorbing regions. Optionally, said absorbing regions arearranged along an axis of said waveguide. Alternatively, said absorbingregions are arranged in a trans-axial direction of said waveguide.

In an exemplary embodiment of the invention, said multiple absorbingregions have same absorption characteristics. Alternatively oradditionally, at least one of said multiple absorbing regions has adifferent absorption characteristics from another one of said regions.Alternatively or additionally, at least two of said multiple regions atleast partially overlap. Alternatively or additionally, at least one ofsaid multiple regions is selectively addressable to control a directionof said ultrasonic waves. Alternatively, at least one of said multipleregions is selectively addressable to control a frequency of saidultrasonic waves.

In an exemplary embodiment of the invention, said waveguide is anoptical fiber.

In an exemplary embodiment of the invention, said absorbing region hassharp boundaries. Alternatively, said absorbing region has at least oneblurred boundary.

There is also provided in accordance with an exemplary embodiment of theinvention, a method of designing an ultrasonic generator powered byelectromagnetic radiation, comprising:

-   -   determining a desired property of a generated ultrasonic wave;        and    -   calculating a spatial absorbing profile of at least one        transduction region of said generator to achieve said desired        property.

There is also provided in accordance with an exemplary embodiment of theinvention, a method of designing an ultrasonic generator powered byelectromagnetic radiation, comprising:

-   -   determining a desired property of a generated ultrasonic wave;        and    -   calculating at least one of a geometric characteristic and a        physical characteristic of at least two transduction regions of        said generator to achieve said desired property. Optionally,        said geometric characteristic comprises a length of at least one        of said regions. Alternatively or additionally, said geometric        characteristic comprises a spacing between said regions.        Alternatively or additionally, said geometric characteristic        comprises a number of said regions. Alternatively or        additionally, said physical characteristic comprises an optical        density of at least one of regions. Alternatively or        additionally, said physical characteristic comprises a        uniformity of density of at least one of regions. Alternatively        or additionally, said property comprises a characteristic        wavelength for a given driving scheme. Alternatively or        additionally, said property comprises a characteristic        wavelength power spectra, for a given driving scheme.        Alternatively or additionally, said property comprises a spatial        propagation profile, for a given driving scheme. Alternatively        or additionally, said property comprises a characteristic        acoustic envelope for a given driving scheme. Alternatively or        additionally, said calculating is performed prior to manufacture        of said generator. Alternatively or additionally, said        calculating is performed after manufacture and prior to use of        said generator. Alternatively or additionally, said method        comprises effecting at least one of said characteristics by        selecting an irradiation wavelength of said absorbing areas.        Alternatively, said method comprises effecting at least one of        said characteristics by optically activating at least one of        said absorbing areas.

There is also provided in accordance with an exemplary embodiment of theinvention, an acoustic generator, comprising:

-   -   a source of electro-magnetic radiation; and    -   a plurality of waveguides coupled to said source, each waveguide        defining an absorbing region that converts said radiation into        an ultrasonic acoustic field,    -   wherein said source irradiates at least two of said plurality of        waveguide at a same time such that fields of said two waveguides        interact. Optionally, said generator comprises a controller,        coupled to said source and operative to selectively control each        of said acoustic fields. Optionally, said controller sets a        relative phase between said two fields.

In an exemplary embodiment of the invention, said controller sets arelative pulse rate between pulsed light provided in said twowaveguides. Alternatively or additionally, said controller sets arelative pulse phase between pulsed light provided in said twowaveguides. Alternatively or additionally, said controller sets arelative amplitude between said two waveguides.

In an exemplary embodiment of the invention, said fields interact toobtain a desired propagation direction. Alternatively or additionally,said fields interact to enhance power in a certain wavelength.

There is also provided in accordance with an exemplary embodiment of theinvention, an ultrasonic generator, comprising:

-   -   a source of electro-magnetic radiation that generates radiation        having a plurality of propagating components;    -   an electromagnetic waveguide; and    -   an absorbing region in said waveguide that converts incident        electromagnetic radiation into ultrasonic waves, wherein only        one of said components interacts with said absorbing region to        create ultrasound. Optionally, a second one of said components        interacts with said waveguide other than at said absorber to        generate ultrasound. Alternatively or additionally, said second        generated ultrasound has an intensity high enough to attack        adjacent plaque in a blood vessel.

In an exemplary embodiment of the invention, said generator comprises anoptical acoustic detector in said waveguide and wherein an additionalone of said components interacts with said waveguide to detect anambient ultrasonic field.

In an exemplary embodiment of the invention, a second one of saidcomponents exits said waveguide at a high enough power to interact within-vivo biological tissue. In an exemplary embodiment of the invention,said different components have different polarizations. Alternatively oradditionally, said different components have different wavelengths.

There is also provided in accordance with an exemplary embodiment of theinvention, an ultrasonic probe, comprising:

-   -   a waveguide having an axis along which electromagnetic radiation        propagates and defining an absorber that converts said radiation        into forward propagating ultrasound that further propagates in a        general direction of said axis; and    -   an output port that outputs light carries in a same direction as        said ultrasound. Optionally, said output port is formed in said        waveguide. Alternatively or additionally, said probe comprises a        forward looking ultrasonic detector defined in said waveguide.

There is also provided in accordance with an exemplary embodiment of theinvention, an acoustic generator, comprising:

-   -   a source of electro-magnetic radiation;    -   a waveguide coupled to said source; and    -   a plurality of spaced apart absorbing regions defined in said        waveguide,    -   wherein each of said absorbing region converts said radiation        into an ultrasonic acoustic field.

In an exemplary embodiment of the invention, said waveguide is flexible.Alternatively or additionally, said waveguide is rigid. Alternatively oradditionally, said waveguide is formed into a guidewire. Alternativelyor additionally, said waveguide is formed into a catheter. Optionally,said catheter is a balloon catheter.

BRIEF DESCRIPTION OF THE FIGURES

Particular embodiments of the invention will be described with referenceto the following description of exemplary embodiments in conjunctionwith the figures, wherein identical structures, elements or parts whichappear in more than one figure are preferably labeled with a same orsimilar number in all the figures in which they appear, in which:

FIG. 1 is a schematic illustration of an ultrasound generating opticalfiber, in accordance with an exemplary embodiment of the invention;

FIG. 2A is a schematic illustration of an ultrasound generating opticalfiber in accordance with an alternative embodiment of the invention;

FIG. 2B illustrates the absorption of energy in the embodiment of FIG.2A as modified by reflection, in accordance with an exemplary embodimentof the invention;

FIG. 2C illustrates the absorption of energy in an exponential absorber,in accordance with an alternative exemplary embodiment of the invention;

FIG. 2D illustrates the absorption of energy in a discrete-stepabsorber, in accordance with an alternative exemplary embodiment of theinvention;

FIGS. 3A and 3B illustrate the effect of using two side-by-side opticalfibers on the resulting acoustic field pattern, in accordance with anexemplary embodiment of the invention;

FIG. 4 illustrates a single optical fiber with multiple light absorbingareas, in accordance with an exemplary embodiment of the invention;

FIG. 5 illustrates an optical ultrasonic system, in accordance with anexemplary embodiment of the invention;

FIG. 6 illustrates the use of a fiber-optic ultrasound source as aguidewire, in accordance with an exemplary embodiment of the invention;

FIG. 7 illustrates the use of a fiber-optic ultrasound source forultrasonically marking an invasive tool, in accordance with an exemplaryembodiment of the invention;

FIG. 8 illustrates a multi-element probe, in accordance with anexemplary embodiment of the invention; and

FIG. 9 is a graph illustrating experimental results of a deviceconstructed in accordance with an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 is a schematic illustration of an ultrasound generating opticalfiber 100, in accordance with an exemplary embodiment of the invention.Fiber 100 includes a body 102 through which a pulse (or train of pulses,or another waveform such as a saw-tooth or Gaussian form) ofelectro-magnetic radiation 104 (indicated by an arrow), for exampleinfra-red, ultraviolet or visible light, propagates. At least some ofthe illumination is absorbed by an absorber 106, thereby heating it andcausing it to expand abruptly and emit an ultrasonic wave. This wave istypically a multi-spectral wave. As explained in the following, however,the spectrum and/or direction of the wave may be manipulated.

Potential advantages of using guided-volumetric absorption are:

-   -   (a) The generating radiation can be guided through the        absorption process and is thereby confined laterally. Lateral        spreading of the generating wave through the absorption process        as would occur in unguided situations where the beam diffracts        and expands, can generally be prevented. The radiation power        density is therefore diminished only due to the absorption        process and not as a result of beam-spreading;    -   (b) The absorption can be spread over a greater depth of the        target and can therefore generate a more controlled ultrasonic        wave; and/or    -   (c) The use of volume absorption allows for potentially better        control of the resulting acoustic waveform, for example by        variation in the degree of absorption within the absorbing        region.

The ultrasonic wave generated in the absorbing region is essentially theshock wave generated by thermal shock due to the abrupt heating of theabsorbing medium. The characteristics of the acoustic signals generatedusing this thermo-elastic regime possibly derive primarily from thetemporal characteristics of the deposited electro-magnetic energy and/orfrom the geometrical form of the heat deposition the heat-dissipationproperties of the surrounding medium. For simplicity, various effects,such as the convection and radiation of heat away from the heated regionand the direct coupling of the acoustic and electro-magnetic phenomenon,are neglected. Also for simplicity, only the initial acoustic signal,before it is distorted by traveling through the surrounding medium, isconsidered, and only the contribution due to the linear response of thematerial is included. It should be clear that none of these assumptionsand/or limitations are critical for actual operation of the inventionand they are provided only for simplifying the presentation and forsimplified initial calculation.

Under these assumptions the displacement of the generated ultrasound canbe represented as:u _(k)(X,t)=α_(T)(3λ+2μ)∫_(V)Θ(ξ,t)δ_(ij) *G _(ki,j)(ξ,t;X,0)dV  (1)Where

-   u_(k)(X, t) is the ultrasonic displacement in the three    orientations, k.-   α_(T) is the linear thermal expansion coefficient of the material-   (3λ+2μ) are the Lame constants of the material-   Θ(ξ, t) is the instantaneous heat distribution across the heated    region-   δ_(ij) is the Kroneker delta function-   * denotes convolution in time-   G_(ki,j)(ξ, t; X, 0) is the derivative of the Green's function in    the j direction    and the integration is performed over the entire heated region. As    will be described below, the heated region may be non-uniform or    discrete. Alternatively or additionally, for example as described    below, even a uniform region can be heated in a non-uniform manner,    for example by using wavelength addressing to selectively address    different parts of an absorbing region with different energy levels.

The frequency response of the absorber includes various spectralcomponents, as described below, for simplified cases. In a practicalimplementation, the spectral components may be somewhat different,however, the following discussion may be used as an aid in defining thenumber and other properties of absorbing areas, in accordance withexemplary embodiments of the invention.

The leading-edge of the Green's functions for displacements ischaracterized by an abrupt step singularity (Pekeris in Proc. Acad.Sci., 41, pp. 469-480 and pp. 629-639, 1955), causing the leading edgeof ultrasonic signal reflect the temporal distribution of the depositedelectro-magnetic pulse.

Taking a typical laser pulse with a rise-time on the order of 10nanoseconds, and, for example, a glass material (for body 102) with arelatively poor heat conduction, the thermal shock, and the resultingacoustic disturbance corresponds almost entirely to the laser-pulsetransients, and the initial acoustic wave comprises of the frequencyspectrum resulting from a transient excitation of 10 ns. This is abroad-band excitation with a center-frequency on the order of 30 MHz.

The temporal width of the longitudinal component of the ultrasound, asobserved in the Green's functions, is on the order of less than 0.01r/c, where c is the ultrasonic velocity and r the distance of the sourcefrom the observation point. For example, if a Gaussian laser pulse of 10ns width is used for the generation in glass at a distance of 1 mm, theleading edge of the ultrasonic pulse would be on the order of 5 ns.Similarly, for this distance, the contribution of the width of Greens'sfunction is approximately 0.01×1 mm/6,000 m/s=1.7 ns, so the pulse widthfor a point-source generator is on the order of the electro-magneticpulse width. It is expected that a bi-polar pulse be generated, thecontribution is in the form of the derivative of Green's function.

Taking into account the volume of the generator, the temporal shape ofthe initial ultrasonic pulse may be characterized by the convolution ofthe electro-magnetic pulse shape and the geometry of the heat source,either one of which may be controlled and/or designed, in accordancewith exemplary embodiments of the invention. Considering a square sourcecross-section of width of 1 mm, one obtains an ultrasonic wave with twomain features—the bipolar pulse ensuing from the edges of theilluminated region, with a width commensurate with that of theelectro-magnetic pulse, and a residual ultrasound pulse corresponding tothe width of illuminated region (this is due to any asymmetry in thebi-polar Green's function derivative). Consequently two major frequencycomponents are observed—a pulse with a time width comparable to thewidth of the electro-magnetic pulse, and a central component with awavelength comparable to the width of the heated volume.

For example, a single region of width w is expected to generateultrasound with a central frequency for which w corresponds to half anacoustic wavelength. For example, in glass, with acoustic velocity ofnearly 6,000 m/s, a uniformly illuminated absorbing region of breadthw=½×6,000/30 MHz=0.1 mm (and odd multiples thereof) reinforces the firstwavefront ensuing from the first thermal shock front with the wavefront,of opposite sign, ensuing from the opposite edge of the thermal shockfont. In another example, for a glass target and 1 mm illumination, thiscorresponds to a central frequency of f=2 c/w=2×6,000 m/s/1 mm=12 MHz.The relative strength of this component as compared to that ensuing fromthe edges depends, inter alia, on the thermal gradient at the boundaryof the thermal source: the sharper this gradient the stronger thecontribution of the edge component in the signal; conversely as thethermal boundary becomes more gradual or blurred, the lower frequencycontribution of the width of the source increases in importance. Asdiscussed below, the attenuation of the generating electromagneticradiation as it travels along the absorbing region, introduces a gradualboundary to the region and effectively strengthens the relativelow-frequency component generated. Maintaining the absorbers short(small values of w), as drawn in FIG. 1, enhances the relative strengthof the higher frequency components in the generated ultrasound.

In an exemplary embodiment of the invention, at least one additionalabsorbing region 108 is provided distal of absorber 106 to absorb atleast some of the light (if any) that is not absorbed by absorber 106.In an exemplary embodiment of the invention, the distance between theabsorbers, a, and their extent in the axial direction, w, serve todesign the desired ultrasonic characteristics of the resulting waves asdiscussed below.

In an exemplary embodiment of the invention, a low frequency componentis generated by increasing the length of the absorbing region in thefiber and/or using a series of suitably spaced heating regions. Forexample, to generate a 600 KHz acoustic signal, a series of regions oflength w=λ/2 and a similar spacing can be used. In a glass waveguide,λ/2=½ c/f=½×6,000 m/s/600 KHz→regions 5 mm in length may be used. Ingeneral, the spatial distribution of heated volume is related to aFourier Transform of the resulting spectrum, depending on the envelopeof the illumination. Increasing the number of absorbers narrows thewidth in the Fourier plane and the resulting spectrum of the signal. Asthe boundaries of the absorbing regions are made more gradual thehigh-frequency components are reduced. Similarly, introducing amonotonically changing region spacing and length results in atime-variable spectrum or chirp signal. Consequently, to reinforce aparticular frequency component in the generated acoustic wave, thespacing, a, between the absorbers has to correspond to the acousticwavelength of that component. This is shown schematically in FIG. 1where absorbers 106 and 108 are spaced by λ—the acoustic wavelength.

If thin absorbing volumes are used, they may each generate a very highintrinsic acoustic frequency, as determined by their geometrical widthand the rise-time of the generating electromagnetic pulse. For example,for a 10 ns rise-time pulse, and an absorber that is narrower than say0.01 mm, can give rise to ultrasonic components at 300 MHz or more. Thedistance between the absorbing regions determines a lower frequency,with a generally lower power. If, as in FIG. 2A the absorbing region iswide, the lower frequency component is stronger. Optionally, the lowerfrequency component is made to dominate the waveform by providing agradual change of absorption in at least part of the absorbing regionboundary. In this manner the edge effects are subdued and the volumetriceffects over the extent of the absorbing region, dominate. In anexemplary embodiment of the invention, the boundary area may comprises alinear increase in optical density over a length that is, for example,1%, 5%, 10%, 20% or any smaller, intermediate or greater percentage ofthe length of the absorbing region.

Optionally, a reflector 110 is provided distal of absorber 108, forexample, at a tip of fiber 100. This reflector returns light that passedabsorbers 106 and 108, to be absorbed by the absorbers. Alternatively,absorber 108 is a total absorber of all the light and reflector 110 canbe omitted. In an exemplary embodiment of the invention, to reinforce aparticular frequency component in the generated acoustic wave, thedistance between the last absorber 108 and the reflector 110 shouldcorrespond to half the acoustic wavelength of that component. This isshown schematically in FIG. 1 where absorber 106 and 108 are spaced byand acoustic wavelength, a=λ, while the distance between absorber 108and the reflector 110 is half that value, a/2=λ/2.

In the reflector embodiment, the acoustic signal will have two sets ofsuper-imposed components, two due to the absorption of electro-magneticwave on the forward travel, and two due to the backward travel of theelectro-magnetic after reflection from the tip of the waveguide which isfully reflective; as the speed of electro-magnetic radiation is verymuch larger than that of the ultrasound, the two sets of acousticwaveforms super-impose, optionally compensating for the decay of theincident electro-magnetic power with distance. The second absorptionregion receives reduced incident power due to the absorption in thefirst region, but on the return pass the situation is reversed. As theabsorbing regions are suitably spaced, and their degree of absorptioncan be controlled, the relative intensities of the four components inthis case can be designed to suit the application. One potentialadvantage of this approach is the ability to generate a unique acousticwaveform that can be readily identified by its specific characteristicsin the system. Another potential advantage of this approach is theability to generate a more uniform acoustic waveform as compared toother arrangements where the acousto-optic interaction is confined to asmall region or a boundary layer.

In an exemplary embodiment of the invention, the absorbing regions aredichroic, permitting the transfer of a second electro-magneticwavelength. As noted below for various embodiments, this allows thesize, number, location and/or intensity of the absorbing regions to becontrolled in real-time or prior to use of the system, by havingparticipating absorbing volumes being selected by wavelength. Thecombination of source parameters including the dimensions of theabsorbing regions, the degree of absorption and the distribution of theabsorption profile within the absorbing region, the separation of theregions and the intensity and rise-time of the generating radiationpulse or pulses, controls the characteristics of the ensuing ultrasonicwaveforms. It is thereby possible, by judicious choice of the aboveparameters to control the directionality and direction, the frequencycontent, the overall envelope and the intensity of the generated signal,by design and/or by selective manipulation of various illuminationparameters.

As noted above, the relative absorption properties of absorbers 106 and108 and/or the reflective properties of the mirror may be used toachieve a desired spatial absorption profile in the fiber. Optionally,for the same or a different purpose, at least one of the absorbers doesnot cover the entire cross-section of the fiber, to allow apredetermined portion of the light to pass and possibly be absorbedand/or reflected at a later time. Alternatively, the absorbing area ispolarization dependent, for example itself acting as an absorptionpolarizer, so that it only absorbs one component of light polarization.An absorber may be one or more of dichroic, polarization dependent andspatially varying in the cross-sectional direction.

In an exemplary embodiment of the invention, the absorbing regions aredefined inside the fiber, for example, by doping a material (e.g.,glass) of which the fiber is made or by introducing deliberate damage,applying stress or otherwise modifying the material continuum oruniformity. Alternatively or additionally, the fiber is cut and splicedwith an absorbing fiber section (e.g., a colored or polarizing fiber)and/or an absorbing material section, for example a plate coloredmaterial or a linear polarizer, which are optionally coated with acladding. For example, for near-IR radiation doping with and absorbersuch as CuSO4 produces the desired absorption region. This may beintroduced into the fiber by splicing an undoped fiber with section of asimilar fiber with such doping.

For clarity, cladding of fiber 100 is not shown in FIG. 1. In someembodiments of the invention, absorption is provided in the cladding,for example, by replacing a section of the cladding with an absorbingmaterial. Alternatively or additionally, the refractive index of thecladding is modified to allow some light to leak out and be absorbed byan absorber outside of the fiber. A potential advantage of this type ofmechanism is that some patterns of absorbing regions may be easier tomanufacture outside of a fiber. Such a change in the cladding may,however, cause dispersion problems in the fiber, which are expected tobe insignificant in many cases.

FIG. 2A is a schematic illustration of an ultrasound generating opticalfiber 200 having a body 202, in accordance with an alternativeembodiment of the invention. Unlike fiber 100 (FIG. 1), fiber 200utilizes an extended absorber 206 that has a length close to λ/2 (halfthe desired central acoustic wavelength) to maximize the generation ofthe desired acoustic frequency component. Other lengths may be used aswell and depend, inter alia, on the existence of a nearby fiber endand/or a reflector. In an exemplary embodiment of the invention, a lightpulse indicated by an arrow 204 is absorbed along absorber 206.Optionally, a mirror 210 is provided to reflect unabsorbed light backalong absorber 206. The length of the absorber here can approach λ/2.Using the same parameters as before, the length of the absorber wouldnow be some w=½×6,000/600 KHz=5 mm for generating a strong 600 KHzcomponent. Note that, although FIG. 2A shows region 206 at the tip ofthe fiber, it can equally well be located at a distance from the fibertip.

In an exemplary embodiment of the invention, for example, in fiber 200or in fiber 100, multiple reflections through the absorbing regions areprovided to make the generating region more uniformly excited. In oneexample, the arriving wave 204 is passed through a polarized beamsplitter 212 and then through a quarter wavelength plate 214. Inoperation, incident light in one polarization, is transmitted throughbeam splitter 212, rotated 45° by wave plate 214 to form circularlypolarized light and on reflection from the mirror at the end of thewaveguide, rotated again to become incident on splitter 212 in anorthogonal polarization state. Therefore the incident beam traverses theabsorbing region twice before it is rotated to the original polarizationand leaves the volume. In some embodiments of the invention, the fiberitself is made with special polarization properties, for example, beingpolarization preserving.

This reflection method reduces somewhat the non-uniformity found inrelatively large absorbing regions due to the decay of the illuminationas it propagated. When the illumination is reflected to travel againthrough the absorbing region the absorbed intensity on the forward passdecays in the forward direction while the absorption on the reverse passdecays in the opposite attitude thereby forming a more uniform overallacoustic energy source. This is useful for example, for lower-frequencygeneration where the length of the absorbing region corresponds to thedominant acoustic wavelength generated. Lower frequency US may be usedfor ablation of plaque and unwanted tissue where the incident energy hasto be designed to generate sufficient cavitations or mechanicalresonance of the target; typically lower frequencies are used for thispurpose.

FIG. 2B shows the effect of reflection on the uniformity of energyabsorption. Reference 220 shows forward and backward propagating light222 and 224 (in a two pass example). Reference 230 is a graph showing,super imposed, relative forward radiation absorption 232, relativebackwards radiation absorption 234 and total radiation absorption 236.The total absorption corresponds to the actual intensity of emittedultrasonic radiation.

Optionally, the density of absorber 206 varies in a manner that takesinto account the reduction wave amplitude and/or reflection, so thatthermal heating is uniform or has a different desirable form. Forexample, to generate a side-looking component at an off-perpendiculardirection, a decaying distribution can be used. Another example is asinusoidal absorption characteristic (whether strictly or piece-wisesinusoidal) for reinforcing the generation of a certain acousticfrequency.

FIG. 2C illustrates the absorption of energy in an absorber having anexponential absorption coefficient, in accordance with an alternativeexemplary embodiment of the invention. Reference 240 shows absorber 206within a waveguide with an exponentially graded absorption 244, forabsorbing forward traveling light 242. A graph 250, shows an absorptiondensity 252 increasing exponentially, so that when interacts with theactual beam, the result is uniform absorption of energy 256 alongabsorbing region 206 and therefore a relatively uniform energydistribution.

The uniformly varying absorption profile of FIG. 2C may be relativelydifficult to manufacture. In an exemplary embodiment of the invention,the exponential profile is approximated by a discrete series ofindividual absorbers, each with a possibly uniform absorption profileand adjacent or spaced apart. FIG. 2D illustrates the absorption ofenergy in a discrete-step absorber, in accordance with an alternativeexemplary embodiment of the invention. Absorber 206 comprises aplurality of absorbers 264, each with a different absorptioncoefficient, for example, with an exponential increasing coefficientbetween the absorbers. Although the absorbers are shown in contact witheach other, they may be spaced, for example by an absorption-freewaveguide portion. A graph 270 shows a piece-wise approximation to theexponential absorption profile 272, with a resultant energy deposition276 that is substantially spatially uniform, e.g., with smallvariations.

In an exemplary embodiment of the invention, only a small number, suchas 2, 3 or 4 absorbers are provided, for example. Reflectors may beprovided, of course in the embodiments of FIGS. 2C and 2D.Alternatively, a larger number of absorbers, such as 10, 20 or anyintermediate smaller or larger number, may be provided.

The apparatus described above can be used to generate ultrasound formany different applications, of which several examples are: ultrasonictreatment; ultrasonic ablation; indirect heating using ultrasound;sonophoresis; ultrasonic monitoring of various parameters, such asthickness or depth; ultrasonic characterization of a target materialand/or for imaging; and photo-acoustic imaging or characterization of atarget material. Optionally, as described below, a plurality ofdifferent ultrasound sources are provided.

In an exemplary embodiment of the invention, the light source is laserlight, optionally, from a wavelength tunable laser. We note that thechoice of laser light is a matter of convenience only and since, in someembodiments, there is no requirement of the coherence of the source, aflash lamp or other optically gated light sources are viable alternativesources.

In an exemplary embodiment of the invention, the absorbers arewavelength selective. For example, laser treatment light passes throughsubstantially unaffected while laser for ultrasound generation isabsorbed. Alternatively to treatment, the transparency to somewavelengths may be used for optical operations, such as providing lightillumination and/or detecting light.

In an exemplary embodiment of the invention, ultrasound detection usesacousto-electric or peizoelectric transducers (not shown) mounted nearthe tip of fiber 100. Alternatively, optical means are used to detectacoustic waves. In an exemplary embodiment of the invention, acousticsignals are detected using an opto-acoustic interaction with anacoustically sensitive optical material provided in the fiber. In anexemplary embodiment of the invention, a detection beam travels throughthe fiber and passes through an acoustic sensitive material incorporatedin or adjacent to a reflector at the tip of the fiber. The acousticallysensitive material may be the same material used for ultrasoundgeneration or it may be separate. In an exemplary embodiment of theinvention, a birefringent material (not shown) is provided nearreflector 110 as a detector so that a reference beam of light having awavelength not absorbed by the absorbers, is affected by changes in thebirefringence that are dependent on stress in the fiber (e.g., stressfrom externally impinging acoustic waves). Alternatively, the fiber as awhole may be birefringent whether by design or inadvertently by theproduction processes. Alternatively or additionally, other opticaldetection methods may be used, to demodulate the effect on the sensingwavelength or wavelengths, for example, such as Fabry Perot resonator,Polarimetric measurements, Interferometry of various types (e.g.,homodyne, heterodyne, speckle, Fucou, Sagnac, holographic),Bragg-grating spectral analysis and/or other optical demodulatingmethods known to the art. The demodulation can be implemented entirelywithin the fiber, or optionally, some or all of the demodulation meanscan be situated external to the fiber, for example, in a controllerexternal to the fiber (e.g., controller 506 described below).

Alternatively or additionally, the boundaries of the absorbing regionsact as partial reflectors that are displaced by the impinging acousticwaves. This displacement generates an inference pattern in the detectionlight, which may be read out, for example, by the controller usingoptical demodulation techniques and/or signal processing methods knownin the art. Alternatively or additionally, reflector 110 may be moved bythe acoustic waves, to modulate the sensing wavelength, for example bygeneration of an interference pattern. In an exemplary embodiment of theinvention, the displacement and/or compression of the tip of the fiberwhich is immersed in an acoustic field is detected by its effect on adetection wave that is reflected from the fiber tip. The reference waveused for detection may pass through the absorbers (completely orpartially) or it may be reflected before the absorbers, for example, bybeam splitter 212 (e.g., having a different polarization), thus allowinga same wavelength to be used for generation and detection. Detection maybe provided at one or more other points along the fiber in addition toor instead of the fiber tip. An alternative to a reflecting surface is areflecting grating or phase array or scattering array that may beimpressed into the fiber material by a variety of methods, including,for example, laser etching.

The shape, location and/or activation of the absorbing regions in one ormore nearby fibers can be used to achieve various effects, especially,beam aiming, enhancement of a particular spectral component within thegenerated ultrasound and/or otherwise selecting a frequency spectrum.

FIGS. 3A and 3B illustrate the effect of using two side-by-side opticalfibers on the resulting acoustic field pattern, in accordance with anexemplary embodiment of the invention. Such multiple fibers are drivenas a phased array, in some embodiments of the invention. In otherembodiments of the invention, the fibers are driven as a mono-pulsesystem as explained below. FIG. 3A (reference 300) shows a side-view oftwo fibers 302 and 304, for example of the type shown in FIG. 1 or inFIG. 2. The two fibers are separated by a distance L, which may beconstant or vary along the ultrasound emitting areas. FIG. 3B (reference306) is a front view of the two fibers. In the example shown, the twofibers are driven in phase, so that the main lobes of the generatedacoustic waves are directed along the normal to the centerline of thefiber array at 0° and 180°. Only the 0° lobe is shown, for clarity.Other relative phases effect other beam directions. In an exemplaryembodiment of the invention, one of the lobes is blocked, for example,by an absorbing material 310 (depicted in the figure as a block of thelobe at 0°), so that essentially one, directional beam ensues from sucha two-fiber probe assembly. Alternatively or additionally, part of alobe may be blocked. Alternatively or additionally, a plurality offibers are arranged in an array, for example, a two dimensional array,such as a hexagon or a linear array, allowing a finer control over thebeam direction. Optionally, the different fibers of the array are drivenwith controlled light intensities to effect simultaneous phase and/oramplitude control.

In an exemplary embodiment of the invention, the multiple fibers areused for phased-array type or mono-pulse type detection of acousticfields. In mono-pulse detection, the field at each fiber is detectedseparately and/or each source is activated separately and then theresults are processed together. In the two-sensor example of FIG. 3,this amounts to three measurements—one with the first fiber only, onewith the second fiber only, and one with both fibers activatedsimultaneously. As the ultrasonic beam patterns differ for each of thesemeasurements (e.g., due to their covering different areas/angles), atarget reflects at different intensities in each measurement. Thedifferences in the measured intensities be can related back to obtaininformation on the spatial location of the target, for example usingmethods well known in the art of radar.

In another example, ultrasonic beam directivity is obtained byintroducing fibers with preferred ultrasound emission directions, forexample using absorbing cladding covering most of the angular range ofeach fiber except for a specific designated emitting angular window.Active sweeping may also be obtained, for example, by changing the phasedifference between fibers in a fiber pair. Various directionalityproperties may also be achieved by varying the relative intensity of theirradiation of the two fibers.

FIG. 4 illustrates a single optical fiber 400 having a body 402 withmultiple light absorbing areas 404 (e.g., 2, 3, 4, 5, 6 or more regions)and an optional tip region and/or reflector 406, in accordance with anexemplary embodiment of the invention. In an exemplary embodiment of theinvention, the absorbing areas are selective to different wavelengths.Thus, the location of ultrasound emission is dependent on the wavelengthused. Alternatively or additionally, multiple absorbing areas areexcited, to provide relatively long ultrasonic sources (e.g., for heattreatment or for generating low frequencies). Alternatively oradditionally, multiple wavelengths are used simultaneously, possibly atdifferent pulse rates and/or relative phases. Thus, a plurality ofultrasound sources can be created at desired relative phases and pulserates, allowing various interactions between the sources to be provided.Alternatively or additionally, the signals from these sources can bedistinguished during detection, possibly using a single detector, forexample, based on different pulse repetition rates, pulse envelopesand/or frequencies of the different sources.

In an exemplary embodiment of the invention, by selecting the locationof excitation, the direction of a beam, relative to the axis of thefibers, in a multiple-fiber arrangement can be controlled. Alternativelyor additionally, each location 404 is a polarization dependent absorber(e.g., a polarizer) and the ultrasonic source location is selected bychanging the polarization alternatively or additionally to changing thewavelength. For example, if two absorbers with perpendicularpolarization axes are provided, sending light with the polarization ofthe first absorber, will allow the light to pass the first absorber andbe absorbed by the second. The absorbers may also be wavelengthdependent and/or have non-perpendicular polarization axes.

In some embodiments of the invention, various “addressing” schemes maybe used, in which certain pulses are directed to certain absorbingregions, based on previous pulses. For example, if photo-activatedabsorbers are provided, one wavelength (e.g., ultraviolet) can be usedto “activate” an absorber by changing its absorption characteristics,and a second (e.g., high-energy pulse) will then be absorbed and used togenerate the ultrasound. For example the material sold as “Photogray”,used in sunlight accommodating eye-glasses, can be used.

In another example, the absorber is wavelength-dependent along itscross-section, exhibiting a different behavior on different parts of thecross-section; again, this may be used for beam forming. Alternativelyor additionally, some of the cross-section is transparent to allow lightto pass on along the fiber. Alternatively or additionally, for examplein larger, multimode fibers some of the cross-section absorbs onewavelength of light and some a different wavelength of light. Theregions may have different lengths and/or they may overlap incross-section. It should be noted that providing absorption of differentwavelengths at different sectors of a cross-section is functionallyequivalent, in some applications, to providing multiple fibers.

In a more generalized manner, the interaction between multiple sourcescan be analyzed with respect to two major axes, the radial and theaxial.

In the radial direction the presence of a second ultrasonic source andthe resulting acoustic field corresponds to that of a dipole axialsource. A separation, a, between the sources (as shown in FIG. 1)determines the directionality of the different frequency components ofthe combined generator. It should be appreciated that a is related to aphase difference between the two sources, which may also depend onfrequency and on an imposed phase difference in driving the sources. Asnoted above, phasing the source activation in real-time allows for areal-time variation in the parameters of the acoustic beam, includingfor example an angular sweeping of the beam. The directionality of thecombined elements is often strongly frequency dependent and therefore,since the sources are typically broad-band, a spectral analysis of thedetected components relates to different radial directions of thesystem. In one embodiment of the invention, this information can be usedto generate an image with its circumferential pixel elements beingdetected at different frequencies. Alternatively, frequency division offunctions can be effected. For example, for a simple source, lowfrequencies (e.g., for treatment) propagate perpendicular to the axis ofthe sensor array, while high frequencies (e.g., for imaging and/ortreatment) propagate at an angle to this direction. For example, as thefrequencies increase such that the separation a approaches half anacoustic wavelength, the main ultrasound beam will be directed furtherand further off this direction approaching, at the limit, the directionalong the axis of the array.

Along the axial direction the separation of the individual absorbingregions carries a different significance—source separations in multiplesof an acoustic wavelength will reinforce, while others will destruct;consequently, depending on the number of sources, a frequency and/orspatially narrower band signal is generated at certain pre-determinedfrequencies. As should be appreciated, such a signal can also be steeredin the azimuth direction, in accordance with exemplary embodiments ofthe invention.

In an exemplary embodiment of the invention, the physical and/orgeometrical characteristics of the absorbing regions are designedmathematically, e.g., based on wave generation and propagationequations. Alternatively or additionally they are designed iteratively,using real and/or a simulated model.

FIG. 5 illustrates an optical ultrasonic system 500, in accordance withan exemplary embodiment of the invention. In an exemplary embodiment ofthe invention, a probe 514 comprises at least one optical fiber 516,such as those described above, that includes an ultrasound generatingand/or detecting tip 518. Light for generation of ultrasound and/oroutputting a beam of light at tip 518 is provided by one or more lightsources 508, for example a laser source and/or a flash lamp. In the caseof a flash lamp, a filter with one or more spectral pass regions may beprovided, for generating a desired spectrum.

The light from the sources is then optionally modulated (e.g., toprovide a pulsed source or a different envelope, such as saw-tooth,sinusoidal or one that relates to the desired acoustic waveform) by amodulator and delay source 510. The delay or pulsing phase differencebetween different light beams may be used, for example, to control abeam direction. In some embodiments, the source is self-modulated (e.g.,a pulsed laser).

It should be noted that in many embodiments of the invention a probe 514can comprise only a single fiber, with a relatively small diameter.Optionally, this fiber is coated with various materials, such asanti-coagulants and bio-compatible polymers. Alternatively oradditionally, a hollow waveguide is used.

In some embodiments of the invention, multiple fibers and/or multiplesources are used. In these a coupler or switch 512 may be provided forcoupling the light to probe 514 and couple detection light from probe514 to a detector 504 (if necessary). The generation and detection oflight may be controlled, for example, by a controller 506. Optionally, acomputer (e.g., a microcontroller) 502 is provided, for example, for auser interface and/or for storing recorded signals, images and/or otherdata.

An external device 520, for example, an imager, a sound source and/or atreatment device may be controlled by controller 506. In an exemplaryembodiment of the invention, the imager is used for reconstructing animage based on acoustic radiation provided by probe 514. Suchreconstruction may be, for example, based on detection of transmissionand/or reflection radiation, as known in the art. Alternatively oradditionally, the imager is used to detect the position of probe 514. Anexternal ultrasound source may be used instead of or in addition to asound source in probe 514, with probe 514 being used for detection ofthe sound and providing an image or other information. A separatetreatment device may be controlled by the computer to treat about probe514, for example, to remain aimed at probe 514 and/or using informationor an image from probe 514. Alternatively, manual coordination may beused. The system may also be employed for photo-acoustic imaging wherean independent sensor (possibly utilizing the same or a similar opticalfiber) maps the temperature of the object under test as the probe tip isscanned through various positions.

Depending on the exact implementation, one or more of the followingfeatures may be provided in system 500:

(a) Generation of ultrasonic waves for heating tissue, for example,using lower frequency ultrasound and/or ultrasound generated along asignificant length of probe 514.

(b) Generation of ultrasonic waves for fragmenting plaque, stones orother unwanted tissue. Again, lower frequency ultrasound, possibly in aforward direction, may be used. Suitable frequencies and power levelsare known in the art.

(c) Generation of ultrasonic waves for imaging, e.g., narrow bandwidthor wide bandwidth, of various frequencies.

(d) Generation of a specialized waveform of ultrasonic waves, forexample a train of pulses at well-controlled intervals, or a chirp. Forexample, a series of absorbing regions are spatially spaced in order togenerate the desired temporal behavior of the ultrasonic wave. Forexample a train of ultrasonic pulses is obtained by a sequence ofrelatively thin absorbers. The thickness of the absorbers corresponds tothe width of each pulse and their separation corresponds to the spacingbetween the pulses. Using monotonically varying separations and absorberlengths can generate a chirp waveform.

(e) Provision of a forward- or side-looking (e.g., using an angledmirror in or adjacent the fiber) laser light.

(f) Detection of acoustic radiation.

(g) Usage of fiber 516 as a different type of detector for a variety ofparameters known in the art of optical fiber sensors, for example atemperature sensor, a pressure detector, electric or magnetic fieldsensor or chemical sensor.

(h) Generation of directional or omni-directional acoustic fields, forexample for effecting sonophoresis for enhancing absorption ofpharmaceuticals provided near and/or by probe 514.

(i) Generation and detection of ultrasonic waveforms forcharacterization of the target material or dimensions, for example basedon spectral reflection or other methods known in the art of ultrasoniccharacterization.

(j) Generation of periodic acousto-thermal signals for imaging andcharacterization of a target in methods known in the art ofphoto-acoustic imaging and characterization.

Thus, system 500 (optionally in conjunction with an external device 520)can be used for one or more of the following applications: US plaquefragmentation; laser plaque removal and monitoring; artery dimensionmonitoring; intra-body measurements and imaging (for example usingA-mode and/or Doppler); and/or drug delivery enhancement. Probe 514 canbe, for example, a catheter or an endoscope. In an exemplary embodimentof the invention, probe 514 includes an inflatable distal portion, forexample a balloon, to ensure contact with surrounding tissue and/or tofix the gaze direction of probe 514.

FIG. 6 illustrates the use of a fiber optic ultrasound source 600 as aguidewire, in accordance with an exemplary embodiment of the invention.A guidewire is widely used in coronary procedures and is typicallycharacterized by having a small diameter, and sufficient flexibility tonegotiate the bending in the arteries or other ducts it is introducedinto. While some energy may be lost at small bending radii, this isgenerally not a problem as sufficient energy may be provided fromoutside the body. For this application source 600 is optionally enclosedin a suitable protective jacket. The resulting device may be made ofsimilar dimension as a standard guidewire and handled with the sameprocedure, with the significant advantage of potentially offeringultrasonic sensing. Such sensing may be used, for example, for viewingbranches in blood vessels during navigation and/or for detecting astenosis area, and/or for measuring a vessel's dimensions. Thisadvantage alleviates the need to alternately introduce differentsurgical tools to the treated region, as is state of the art: theguidewire serves to mechanically guide the medical treatmentdevices—such as stent applicators. It can thereby eliminate the need forapplying additional imaging and/or diagnostic tools.

In an exemplary embodiment of the invention, guidewire 600 comprises asingle (or small number) of fibers 602 having one or more absorbingregions 604 defined along its length. Optionally a tip 608, for example,a flexible tip or a different type of tip as known in the art ofguide-wires is provided at a distal end of guidewire 600. Regions 604may be used for generating ultrasound, for example, to be detected on anexternal (or another implanted) imager. Alternatively or additionally,regions 604 are used for imaging sideways or forward and/or fordetecting distances and/or obstructions. In the case of a guidewire,viewing in A-mode, of a single pixel distal of the guidewire tip may beuseful, for example, for detecting forks in vessels, determining a depthof plaque and characterizing its components. Optionally the guidewire isused for measurements for example vessel diameter, wall-thickness andstenosis type and/or thickness, which may be useful, for example, inselecting a suitable stent for implantation.

In an exemplary embodiment of the invention, guidewire 600 is used tocarry a stent and/or a PCTA balloon, which may be mounted on theguidewire or conveyed along it.

Another use of the potentially small profile of a fiber optic acousticsource is using a fiber as a marker or beacon, for example, forindicating a tool on an ultrasound image or for showing its future path.In this use, the fiber is typically used as a beacon, for example apoint beacon or an elongate (e.g., multi-point) beacon. Alternatively,the vibration of the fiber is used to create a Doppler shift in incidentradiation. In an exemplary embodiment of the invention, the wavelengthof generated ultrasound is made to match that of the imaging system(e.g., 520 of FIG. 5) so that the beacon is clearly distinguished.

FIG. 7 illustrates the use of a fiber optic ultrasound source 700 formarking an invasive tool 702, in accordance with an exemplary embodimentof the invention. In the figure, the invasive tool is a hypodermicneedle and the probe passes through the needle possibly without causingsignificant obstruction thereof. In some embodiments of the invention,ultrasound source 700 is used for position determination of tool 702alternatively or additionally to being used for imaging as describedabove. Alternatively or additionally, source 700 is used as a detectorto home in on an acoustic beacon, for example a beacon provided by adifferent implanted fiber. In principle, as ultrasound can traverse thematerial of the invasive tools, such as the needle, the fiber ultrasonicsource can be completely surrounded by the tool, or, as shown in FIG. 7,can be allowed to protrude beyond the tool.

In an exemplary embodiment of the invention, two or more ultrasonicsources are used to better locate the marked tool. If only a pointsource is used the only indication that can be obtained using a simpledetector is the distance to the beacon and the marked tool is known tobe located somewhere on a sphere. By providing two or more sources,positioned a known distance apart, the tool can be positioned at theintersection of the tool length with the two spheres scribed by thedistances measured to either source. This reduces the ambiguity of thelocation, in most practical situation, to a conical surface in 3D space.If, for example, the system tracks the relative motion between theimaging system (e.g., detector) and the sources the ambiguity can bereduced further by the generation of a family of such conical section inspace that cut each other in a decreasing area. Thus, the condition of“physical” continuous motion of the tool offers an unambiguous solutionfor the position of the beacon in space. Alternatively, a plurality ofdetectors or more than two sources may be used. To assist thediscrimination between the beacon signal and the standard imaging signalof the imaging system, the signal from the beacon can be designed toproduce a specialized waveform which can readily be separated from theimaging signals. For example a train of pulses or a chirp, whileessentially at the same frequency as the imaging system, can readily bedistinguished from the imaging signals. Alternatively or additionally, asource that generates different frequencies at different points alongits length may be used and identified (e.g., utilizing differentwavelength selective absorbers with different geometries).

Additional potential advantages of an acoustic-optical transducer inaccordance with exemplary embodiments of the present invention, include:

(a) Transferring significant power to a catheter tip.

(b) Reduced diameter probes.

(c) Ability to be used in strong magnetic fields such as MRI fields.

(d) Avoiding grounding problems, especially when the probe is used underfield conditions.

(e) Simplicity of construction.

(f) Low cost of the active portion of the system, which can be discardedand replaced after each (or a small number of) surgery procedure.

In some embodiments of the invention, an opto-acoustic transducer asdescribed above is used for a multi-element probe, which may, forexample, be used outside the body. FIG. 8 illustrates a multi-elementprobe 800, in accordance with an exemplary embodiment of the invention.In an exemplary embodiment of the invention, probe 800 comprises aplurality of fibers 802 each with an acoustically active tip 804. Thetips are arranged, for example, in a probe body 806. Each fiber may beactivated individually. Alternatively or additionally, the fibers areactivated in concert, for example as a phased-array. In an exemplaryembodiment of the invention, the fibers are powered using a flash lamp,for example, using an electrically controlled LCD to selectively passlight to fibers. Unlike standard piezoelectric transducers, probe 800does not typically require high voltages (or any voltages) at body 806.

The fibers are typically oriented in a linear array, laid side-by side,each fiber generating in the side-looking configuration. The beammanipulation in the plane of the array vector may then be provided byphasing the generation of each fiber-element. The manipulation of theresulting beam in the perpendicular direction may be effected by themultiple generating/receiving elements in each fiber. In this manner atwo-dimensional phased array can be formed. Additionally oralternatively, the fiber sources can be used in the forward-lookingconfiguration. In this option a one- or a two-dimensional array isformed by assembling the fiber tips in a line or a two-dimensionalmatrix, respectively. In this case the beam is optionally manipulated byphasing the transduction of all the array elements. In both of theexamples above, a suitable ultrasonic isolation medium is optionallyprovided to minimize the cross-talk between adjacent elements.

FIG. 9 is a graph illustrating experimental results of a deviceconstructed in accordance with an exemplary embodiment of the invention.This signal was acquired using a single absorbing region transmitterfiber and a polarization-demodulated birefringent fiber receiver. Thedevice is inserted in a lucite tubing filled with saline. The firstsignal relates to the direct acoustic cross-talk between the transmitterand the receiver. The tubing wall generates acoustic signals from itsfront- and back-surfaces. Note the reversal of the signal phase at thefront wall as expected from a low to high acoustic impedance. Forreasons of convenience, a liquid target was used instead of a solidtarget for generating the ultrasound. However, as noted above, this maybe provided in some embodiments of the invention. Generation is effectedwith a laser pulse of 1 μJ, 10 ns rise time at 1,064 nm. Detection witha 532 nm laser, and approximate power of 5 mW. The generation anddetection fibers, both multimode, are positioned about 1 mm apart andsome 5 mm from the wall of the tubing in a side-looking arrangement.Tube wall is approx. 2 mm thick. The frequency of the generatedultrasound is approx. 3 MHz as expected from the generating region used:a gradual boundary liquid region mounted onto the fiber, approximately0.8 mm wide.

While the above description focused on optical fibers, other waveguidesmay be used, for example hollow, lens-series or mirror waveguides forlong wavelength infra red radiation. One possible reason for using suchwaveguides is that a same waveguide is used for generating acousticenergy and for conveying electromagnetic radiation (e.g., RF radiation).Alternatively or additionally, the illuminating electro-magneticradiation may be RF radiation, with the waveguide being of a suitabletype.

The present invention has been described using non-limiting detaileddescriptions of embodiments thereof that are provided by way of exampleand are not intended to limit the scope of the invention. It should beunderstood that features and/or steps described with respect to oneembodiment may be used with other embodiments and that not allembodiments of the invention have all of the features and/or steps shownin a particular figure or described with respect to one of theembodiments. Variations of embodiments described will occur to personsof the art. In addition, some embodiments are described as method or asapparatus, the scope of the invention includes apparatus, for example,firmware, hardware and/or software for carrying out the method and/ormethods for using the apparatus, as well as computer readable mediaand/or communication signals on which such software is stored.

It is noted that some of the above described embodiments may describe abest mode contemplated by the inventors and therefore include structure,acts or details of structures and acts that may not be essential to theinvention and which are described as examples. Structure and actsdescribed herein are replaceable by equivalents which perform the samefunction, even if the structure or acts are different, as known in theart. Therefore, the scope of the invention is limited only by theelements and limitations as used in the claims. When used in thefollowing claims, the terms “comprise”, “include”, “have” and theirconjugates mean “including but not limited to”.

1. An ultrasonic generator, comprising: a source of electro-magneticradiation that generates radiation having a plurality of differentwavelengths; an electromagnetic waveguide coupled to the source; and atleast one absorbing region in said waveguide that converts incidentelectromagnetic radiation of fewer than all the plurality of generatedwavelengths from the source into ultrasonic waves.
 2. A generatoraccording to claim 1, wherein at least one of the wavelengths notconverted by the absorbing region into electromagnetic radiation is usedfor light illumination.
 3. A generator according to claim 1, whereinsaid waveguide is formed into a guidewire.
 4. A generator according toclaim 1, wherein said generator is adapted to be inserted into a body.5. A generator according to claim 1, wherein said waveguide comprises anoptical fiber.
 6. A generator according to claim 5, wherein said fiberincludes a non-acoustic optical fiber sensor.
 7. A generator accordingto claim 5, wherein said absorbing region comprises a segment that isadded to said fiber.
 8. A generator according to claim 5, wherein saidabsorbing region comprises a doping of a core or damage to the core ofsaid fiber.
 9. A generator according to claim 1, wherein said absorbingregion is optically controllable to change at least one of saidcriterion and its absorption.
 10. A generator according to claim 1,wherein said source comprises a laser source.
 11. A generator accordingto claim 1, wherein said source comprises a coupler for a laser source.12. A generator according to claim 1, wherein said source comprises aspectral filter.
 13. A generator according to claim 1, wherein said atleast one absorbing region comprises at least two absorbing regions. 14.A generator according to claim 1, wherein said at least one absorbingregion comprises at least three absorbing regions.
 15. A generatoraccording to claim 1, wherein said at least one absorbing regioncomprises at least four absorbing regions.
 16. A generator according toclaim 13, wherein said at least two regions have same absorbingcharacteristics.
 17. A generator according to claim 13, wherein said atleast two regions have different absorbing characteristics.
 18. Agenerator according to claim 1, wherein said at least two regions haveat least one different absorption selectivity criterion.
 19. A generatoraccording to claim 13, wherein said at least two regions have sameselectivity.
 20. A generator according to claim 13, wherein theabsorption properties of said at least two regions are adjusted so as toachieve a desired effect on said ultrasonic waves.
 21. A generatoraccording to claim 13, wherein said at least two regions are spacedapart to achieve a desired effect on said ultrasonic waves.
 22. Agenerator according to claim 21, wherein said effect is selection of awavelength spectrum.
 23. A generator according to claim 20, wherein saideffect is a selection of a spatial field distribution.
 24. A generatoraccording to claim 21, wherein said effect is a selection of an acousticenvelope shape.
 25. A generator according to claim 1, wherein saidabsorbing region is a volume absorber that absorbs said radiation alongits length in a direction of propagation of said radiation.
 26. Agenerator according to claim 25, wherein said absorbing region hasaxially uniform absorption characteristics, along the axis of saidwaveguide.
 27. A generator according to claim 25, wherein said absorbingregion has axially non-uniform absorption characteristics, along theaxis of said waveguide.
 28. A generator according to claim 27, whereinsaid absorbing region has stepped absorption characteristics, along theaxis of said waveguide.
 29. A generator according to claim 1, whereinsaid absorbing region is a solid absorber.
 30. A generator according toclaim 1, wherein said absorbing region is a fluid absorber.
 31. Agenerator according to claim 1, wherein said waveguide comprises anacousto-optical modulator portion that modulates light waves responsiveto an acoustic field.
 32. A generator according to claim 31, comprisingan optical detector coupled to said waveguide which generates a signalresponsive to said acoustic field.
 33. A generator according to claim32, wherein said optical detector detects radiation that passes throughsaid absorbing region unabsorbed.
 34. A generator according to claim 32,comprising a signal processor that reconstructs an image from saidsignal.
 35. A generator according to claim 34, wherein said image is aone dimensional image.
 36. A generator according to claim 34, whereinsaid image is a two dimensional image.
 37. A generator according toclaim 32, comprising a signal processor operative to reconstruct atissue characterization from said signal.
 38. A generator according toclaim 32, comprising a signal processor operative to reconstruct adistance from said signal.
 39. A generator according to claim 1, whereinsaid source provides at least one wavelength having a high power levelthat passes through said absorbing region substantially unabsorbed. 40.A generator according to claim 1, wherein the source generates radiationhaving at least three different wavelengths.
 41. A generator accordingto claim 1, wherein the source generates radiation having at least fourdifferent wavelengths.
 42. A generator according to claim 1, comprisinga plurality of waveguides arranged in a phased-array and a controllerthat controls said source to activate said array as a phased-array. 43.A generator according to claim 1, wherein said ultrasonic wave isoperative to be steered in space by said generator without moving theabsorbing region.
 44. A generator according to claim 1, wherein saidgenerator comprises only a single waveguide.
 45. A generator accordingto claim 1, comprising an ultrasonic absorber, which spatially shapessaid ultrasonic waves.
 46. A generator according to claim 1, comprisinga controller operative to control said source.
 47. A generator accordingto claim 46, wherein said controller synchronizes an operation of saidgenerator with a separate treatment device.
 48. A generator according toclaim 46, wherein said controller synchronizes an operation of saidgenerator with a separate imaging device.
 49. A generator according toclaim 46, wherein said controller reads out optical signals received viasaid waveguide.
 50. A generator according to claim 1, wherein the atleast one absorbing region comprises a volumetric absorption regionwhich absorbs radiation along its length in a direction of propagationof said radiation.
 51. A generator according to claim 50, comprising areflector for reflecting at least a portion of the light that passesonce through said absorber, to pass at least a second time through saidabsorber.
 52. A generator according to claim 51, comprising a secondreflector for reflecting at least a portion of the light that passestwice through said absorber, to pass at least a third time through saidabsorber.
 53. A generator according to claim 1, wherein a second one ofsaid wavelengths interacts with said waveguide other than at said atleast one absorbing region, to generate ultrasound.
 54. A generatoraccording to claim 53, wherein said second generated ultrasound has anintensity high enough to attack adjacent plaque in a blood vessel.
 55. Agenerator according to claim 1, wherein a second one of the wavelengthsexits said waveguide at a high enough power to interact with in-vivobiological tissue.
 56. A generator according to claim 1, wherein saidwaveguide is flexible.
 57. A generator according to claim 1, whereinsaid waveguide is rigid.
 58. A generator according to claim 1, whereinsaid waveguide is formed into a catheter.
 59. A generator according toclaim 1, wherein the at least one absorbing region converts incidentelectromagnetic radiation of only a single of the generated wavelengthsfrom the source into ultrasonic waves.
 60. An acoustic generator,comprising: a source of electro-magnetic radiation; a waveguide coupledto said source; and at least one volumetric absorbing region defined insaid waveguide, which absorbs radiation along its length in a directionof propagation of said radiation, wherein said absorbing region convertssaid radiation into an ultrasonic acoustic field.
 61. A generatoraccording to claim 60, wherein said absorber is uniformly absorbingalong its length.
 62. A generator according to claim 60, wherein saidabsorber is non-uniformly absorbing along its length.
 63. A generatoraccording to claim 62, wherein said non-uniformity is designed toachieve a certain absorption profile.
 64. A generator according to claim63, wherein said absorption profile is designed to achieve asubstantially uniform energy deposition along said absorber.
 65. Agenerator according to claim 62, wherein said non-uniformity is stepped,defining a plurality of contiguous uniform sub-regions with differentabsorbing characteristics.
 66. A generator according to claim 62,wherein said non-uniformity is stepped, defining a plurality ofnon-contiguous uniform sub-regions with different absorbingcharacteristics.
 67. A generator according to claim 60, comprising areflector for reflecting at least a portion of the light that passesonce through said absorber, to pass at least a second time through saidabsorber.
 68. A generator according to claim 67, comprising a secondreflector for reflecting at least a portion of the light that passestwice through said absorber, to pass at least a third time through saidabsorber.
 69. A generator according to claim 67, wherein said secondreflector is polarization discriminating and comprising a polarizationrotator.
 70. A generator according to claim 60, wherein half a thicknessof said absorption area absorbs less than 80% of light absorbed by saidabsorbing area.
 71. A generator according to claim 60, wherein saidabsorbing region has a non-uniform cross-section.
 72. A generatoraccording to claim 60, wherein said absorbing region does not fill across-section of said waveguide.
 73. A generator according to claim 60,wherein said waveguide guides substantially all radiation provided inwaveguide to said absorbing region.
 74. A generator according to claim73, wherein said guidance comprises guiding said radiation to have asubstantially uniform cross-section along said absorbing region.
 75. Agenerator according to claim 60, wherein said absorbing regionselectively absorbs only some of said radiation.
 76. A generatoraccording to claim 60, comprising a plurality of absorbing regions. 77.A generator according to claim 76, wherein said absorbing regions arearranged along an axis of said waveguide.
 78. A generator according toclaim 76, wherein said absorbing regions are arranged in a trans-axialdirection of said waveguide.
 79. A generator according to claim 76,wherein said multiple absorbing regions have same absorptioncharacteristics.
 80. A generator according to claim 76, wherein at leastone of said multiple absorbing regions has a different absorptioncharacteristics from another one of said regions.
 81. A generatoraccording to claim 76, wherein at least two of said multiple regions atleast partially overlap.
 82. A generator according to claim 76, whereinat least one of said multiple regions is selectively addressable tocontrol a direction of said ultrasonic waves.
 83. A generator accordingto claim 76, wherein at least one of said multiple regions isselectively addressable to control a frequency of said ultrasonic waves.84. A generator according to claim 60, wherein said waveguide is anoptical fiber.
 85. A generator according to claim 60, wherein saidabsorbing region has sharp boundaries.
 86. A generator according toclaim 60, wherein said absorbing region has at least one blurredboundary.
 87. A method of designing an ultrasonic generator powered byelectromagnetic radiation, comprising: determining a desired property ofa generated ultrasonic wave; and calculating a spatial absorbing profileof at least one transduction region of said generator to achieve saiddesired property.
 88. A method of designing an ultrasonic generatorpowered by electromagnetic radiation, comprising: determining a desiredproperty of a generated ultrasonic wave; and calculating at least one ofa geometric characteristic and a physical characteristic of at least twotransduction regions of said generator to achieve said desired property.89. A method according to claim 88, wherein said geometriccharacteristic comprises a length of at least one of said regions.
 90. Amethod according to claim 88, wherein said geometric characteristiccomprises a spacing between said regions.
 91. A method according toclaim 88, wherein said geometric characteristic comprises a number ofsaid regions.
 92. A method according to claim 88, wherein said physicalcharacteristic comprises an optical density of at least one of regions.93. A method according to claim 88, wherein said physical characteristiccomprises a uniformity of density of at least one of regions.
 94. Amethod according to claim 88, wherein said property comprises acharacteristic wavelength, for a given driving scheme.
 95. A methodaccording to claim 88, wherein said property comprises a characteristicwavelength power spectra, for a given driving scheme.
 96. A methodaccording to claim 88, wherein said property comprises a spatialpropagation profile, for a given driving scheme.
 97. A method accordingto claim 88, wherein said property comprises a characteristic acousticenvelope for a given driving scheme.
 98. A method according to claim 88,wherein said calculating is performed prior to manufacture of saidgenerator.
 99. A method according to claim 88, wherein said calculatingis performed after manufacture and prior to use of said generator. 100.A method according to claim 99, comprising effecting at least one ofsaid characteristics by selecting an irradiation wavelength of saidabsorbing areas.
 101. A method according to claim 99, comprisingeffecting at least one of said characteristics by optically activatingat least one of said absorbing areas.
 102. An acoustic generator,comprising: a source of electro-magnetic radiation; and a plurality ofwaveguides coupled to said source, each waveguide defining an absorbingregion that converts said radiation into an ultrasonic acoustic field,wherein said source irradiates at least two of said plurality ofwaveguide at a same time such that fields of said two waveguidesinteract.
 103. A generator according to claim 102, comprising acontroller, coupled to said source and operative to selectively controleach of said acoustic fields.
 104. A generator according to claim 103,wherein said controller sets a relative phase between said two fields.105. A generator according to claim 103, wherein said controller sets arelative pulse rate between pulsed light provided in said twowaveguides.
 106. A generator according to claim 103, wherein saidcontroller sets a relative pulse phase between pulsed light provided insaid two waveguides.
 107. A generator according to claim 103, whereinsaid controller sets a relative amplitude between said two waveguides.108. A generator according to claim 102, wherein said fields interact toobtain a desired propagation direction.
 109. A generator according toclaim 102, wherein said fields interact to enhance power in a certainwavelength.
 110. An ultrasonic generator, comprising: a source ofelectro-magnetic radiation that generates radiation having a pluralityof propagating components; an electromagnetic waveguide; and anabsorbing region in said waveguide that converts incidentelectromagnetic radiation into ultrasonic waves, wherein only one ofsaid components interacts with said absorbing region to createultrasound.
 111. A generator according to claim 110, wherein a secondone of said components interacts with said waveguide other than at saidabsorber to generate ultrasound.
 112. A generator according to claim110, wherein said second generated ultrasound has an intensity highenough to attack adjacent plaque in a blood vessel.
 113. A generatoraccording to claim 110, comprising an optical acoustic detector in saidwaveguide and wherein an additional one of said components interactswith said waveguide to detect an ambient ultrasonic field.
 114. Agenerator according to claim 110, wherein a second one of saidcomponents exits said waveguide at a high enough power to interact within-vivo biological tissue.
 115. A generator according to claim 110,wherein said different components have different polarizations.
 116. Agenerator according to claim 110, wherein said different components havedifferent wavelengths.
 117. An ultrasonic probe, comprising: a waveguidehaving an axis along which electromagnetic radiation propagates anddefining an absorber that converts said radiation into forwardpropagating ultrasound that further propagates in a general direction ofsaid axis; and an output port that outputs light carries in a samedirection as said ultrasound.
 118. A probe according to claim 117,wherein said output port is formed in said waveguide.
 119. A probeaccording to claim 117, comprising a forward looking ultrasonic detectordefined in said waveguide.
 120. An acoustic generator, comprising: asource of electro-magnetic radiation; a waveguide coupled to saidsource; and a plurality of spaced apart absorbing regions defined insaid waveguide, wherein each of said absorbing region converts saidradiation into an ultrasonic acoustic field.
 121. A generator accordingto claim 60, wherein said waveguide is flexible.
 122. A generatoraccording to claim 60, wherein said waveguide is rigid.
 123. A generatoraccording to claim 60, wherein said waveguide is formed into aguidewire.
 124. A generator according to claim 60, wherein saidwaveguide is formed into a catheter.
 125. A generator according to claim124, wherein said catheter is a balloon catheter.